The invention is particularly useful with metal halide arc discharge lamps used for general illumination which comprise an arc tube of quartz having elecrodes at each end and containing a fill of mercury, metal halide and an inert gas for starting purposes, and an enclosing outer envelope or jacket made of glass. The jacket is either evacuated or filled with an inactive gas and generally comprises a bulbous or ellipsoidal main portion which is extended at one end into a tubular neck portion to which is attached a screw type base for accommodating the lamp in a standard socket. The common practice has been to mount the arc tube axially within the jacket and to operate the lamp vertically.
Vertical operation of the arc tube is the preferred mode resulting in higher efficacy and longer life. If the arc tube is inclined out of the vertical, internal convection currents affect the discharge and displace it from the axis, causing overheating in some parts and underheating in others, and the end result is lower efficacy and poorer color rendition. However there are many installations wherein considerations of space and convenience require that the outer envelope of the lamp be mounted horizontally. In recent years, primarily as a result of improvements in color rendition, metal halide lamps have found increasing use indoors and in applications where the ceiling height is limited. Fixtures for such applications mount the outer envelope horizontally in order to save space. This has increased the demand for metal halide lamps able to operate without reduction in efficacy or life when the outer envelope is horizontal.
Mounting the arc tube transversely to the axis of the jacket is difficult because the arc tube is longer than the diameter of the neck. One transverse mounting arrangement is described and claimed in copending application Ser. No. 271,505, filed June 8, 1981, by Paul W. Ernest and assigned to the same assignee as the present invention. It features a hinged attachment of the arc tube to a single long support rod extending the length of the jacket and curving in proximity to the wall and serving also as electrical connector to one electrode. During manufacture the hinges permit the arc tube to be folded substantially in line with the support rod for passage through the neck, and then allow the arc tube to be transversely erected in the bulb and a latch holds the arc tube in its transverse attitude after erection.